Paper-like products have been manufactured from various inorganic materials for a number of years. Most commonly, the starting ingredients for such products have comprised either asbestos or fibrous forms of alumina, basalt, glass, wollastonite, zirconia, etc., bonded together with an inorganic or organic binder. Less frequently, flake-like minerals such as bentonite, mica, and vermiculite have been employed as the papermaking element or as fillers in other papers. None of the papers so produced, however, has exhibited the smoothness, the flexibility, and the mechanical strength of a conventional wood pulp paper, particularly a paper containing a substantial rag content. Such deficiencies have limited the use of inorganic papers to applications where those characteristics are less important, e.g., in thermal and electrical insulation, filtration, and chromatography. Nevertheless, because of the other very desirable properties demonstrated by inorganic papers, especially the electrical and thermal insulating characteristics and the resistance to weathering and chemical attack exhibited by micaceous papers, research has continued to develop ever better inorganic papers. The primary goal of this research has been to develop inorganic papers which retain the desirable chemical and physical properties of the commercially-available inorganic papers, but which would display the smoothness, flexibility, and mechanical strength of conventional wood pulp paper. Board can be made through the stacking of sheets or fragments of paper.
Micas belong to the sheet-silicate group of minerals. Sheet silicates of the mica type are built of two units, viz., a tetrahedral sheet and an octahedral sheet. The former consists of tetrahedra of Si-O linked together to form a hexagonal network such that the bases thereof are coplanar and the apices thereof point in the same direction. This configuration yields a Si:O ratio of 2:5. In contrast, the octahedral sheet is generated through the impingement of two tetrahedral sheets pointing toward each other and crosslinked by the sharing of oxygens by Mg (or Al,Fe) in octahedral coordination. The two octahedral corners not falling in the plane of apical oxygens are occupied by hydroxyl or fluoride ions. It is possible that a composite sheet formed in this manner will be electrically neutral, in which case Van der Waals-type forces bond it to the sheets immediately above and below. More commonly, however, an excess negative charge exists due either to ion substitutions or unoccupied sites (vacancies) or a combination of both. Differences in properties arise both from the degree of charge deficiency as well as the location of the excess charge. Charge balance is restored through the uptake of foreign cations in interlayer positions in 12-fold coordination due to hexagonal rings of oxygens located in the sheets above and below. The structural formula of the resulting species can be generalized as ##EQU1## wherein the Roman numerals refer to ligands surrounding the cations and X, Y, and Z represent cations in the superscripted coordination, their nature being as follows:
______________________________________ Cation Cation Radius Illustrative Examples ______________________________________ X &gt;0.6A Li, Na, K, Ca, Sr, Ba, Pb, NH.sub.4, Rb, Cs Y 0.5-0.8 Mg, Al, Li, Mn, Fe, Zn, Cu, Ni, Co Z 0.3-0.6A Si, Al, B, P, Ge, Be, possibly Mg ______________________________________
Glass-ceramic articles wherein the predominant crystal phase consists of a fluormica are well known to the art. Glass-ceramic articles are commonly highly crystalline, i.e., greater than 50% by volume crystalline, and, because such are derived via the controlled thermal crystallization of precursor glass bodies, the crystals developed therein can be very uniform in size. Moreover, because glass constitutes the parent of a glass-ceramic, the many forming techniques utilized in glass manufacture are equally appropriate in the production of glass-ceramics.
Two basic methods for preparing glass-ceramic articles have been disclosed. The first and most generally used process dates to U.S. Pat. No. 2,920,971, the initial patent in the field of glass-ceramics. As outlined therein, the method comprehends three overall steps. First, a glass-forming batch, normally containing a nucleating agent, is melted. Second, the melt is simultaneously cooled to a glass and an article of a desired geometry shaped thereform. Third, the glass article is subjected to a particularly-defined heat treatment to cause in situ crystallization of the glass. In the preferred practice, the heat treatment is customarily divided into two elements. Thus, the glass article will initially be heated to a temperature within the transformation range or somewhat thereabove to initiate nucleation, following which the nucleated article will be heated to still high temperatures, frequently in excess of the glass softening point, to cause the growth of crystals on the nuclei. Further information regarding the physical characteristics of such glass-ceramics and their means of production can be obtained from a study of U.S. Pat. No. 2,920,971 and U.S. Pat. Nos. 3,689,293, 3,732,087 and 3,756,838 which disclose the manufacture of that type of glass-ceramic wherein a fluormica constitutes the predominant crystal phase.
More recently, glass-ceramic bodies have been prepared from certain compositions through the controlled cooling of a glass melt. The process contemplates four basic steps. First, a glass-forming batch, optionally containing a nucleating agent, is melted. Second, the melt is simultaneously cooled to a temperature within the range of about 100.degree.-300.degree. C. above the annealing point of the glass to cause phase separation and nucleation to take place and a glass body is shaped therefrom. Third, the glass body is exposed to a temperature between the annealing point of the glass and the temperature of phase separation and nucleation to cause the growth of crystals on the nuclei. Fourth, the crystallized body is cooled to room temperature. Products made in accordance with this method have been termed "spontaneous glass-ceramics", Additional information with reference to the physical characteristics of such glass-ceramics and the method of production therefor can be secured from a study of U.S. Pat. Nos. 3,985,531 and 3,985,534 which describe the production of that type of glass-ceramic wherein a fluormica comprises the predominant crystal phase.